1. Field of the Invention
The present invention relates to a Raman optical amplifier, and in particular, relates to a feed forward control type Raman optical amplifier.
2. Description of the Related Art
Recently, in trunk optical transmission systems, a large-capacity transmission system adopting a wavelength division multiplex (WDM) transmission technology has become widespread. A Raman optical amplifier is one of the key devices that support such a large-capacity WDM transmission system.
FIG. 1 shows the basic configuration of a general Raman optical amplifier. In FIG. 1, a backward pumping type Raman optical amplifier in which pumping light is supplied in the direction the reverse of the transmission direction of signal light is shown.
A Raman amplification fiber 1 amplifies input signal light using pumping light generated by a pumping light source 2. This input light is multi-wavelength light or WDM light obtained by multiplexing a plurality of segments of signal light with a different wavelength each. For the pumping light source (LD) 2, a laser diode and the like is used, and the pumping light source 2 generates pumping light with a wavelength shorter than that of the signal light. A WDM coupler 3 is an optical device for multiplexing the signal light and the pumping light generated by the pumping light source 2, and guides the pumping light to the Raman amplification fiber 1. Then, the signal light inputted through the input port is amplified by the Raman amplification fiber 1 and is guided to an output port through the WDM coupler 3.
The operation area of the Raman optical amplifier is mainly the gain-unsaturated area, while generally the operation area of an erbium-doped fiber amplifier (EDFA), which is the most popular in today's optical transmission systems spreads across the gain-unsaturated area and gain-saturated area. In the gain-unsaturated area, if pumping light power is constant, gain is always constant even if the input level of signal light changes. However, in the gain-saturated area, even if pumping light power is constant, gain varies with the input level of signal light. On the other hand, recently, with the advent of a broader-band and higher-power optical communication system, the extension of the operation area (change range of the input level of signal light) has been demanded. As a result, the use of the gain-saturated area in a Raman optical amplifier has been promoted.
If a plurality of segments of signal light is inputted and the total power of the plurality of segments of signal light in the Raman amplification fiber 1 becomes sufficiently high or the band of the signal light becomes broader, power tilt (or power deviation) increases and becomes no more negligible due to inter-signal-light Raman effect (inter-signal-light Raman scattering). Here, “Inter-signal-light Raman effect” is a phenomenon in which signal light with a longer wavelength is amplified by signal light with a shorter wavelength. In this case, signal light with a shorter wavelength works as pumping light for signal light with a longer wavelength. Power tilt means that the output level of each segment of signal light is not flat against wavelength.
In this case, if a Raman optical amplifier is used in a gain-saturated area or the optical power of input signal light is high, the output power of the signal light varies as the input power of multi-wavelength light fluctuates due to the increase/decrease of the number of wavelengths to be multiplexed, even if pumping power is constant. Therefore, in a transmission system presuming that the number of wavelength (or number of channels) increases or decreases during operation, there is the possibility that transmission quality (S/N, etc.) temporarily degrades when the number of wavelengths increases/decreases, unless such gain fluctuations are taken into consideration as design margin or measures are taken to cope with such gain fluctuations in designing.
Pumping light is usually supplied backward to the Raman amplification fiber 1 in order to avoid the degradation of transmission quality due to the polarization-dependence of the gain of signal light, pumping light noise transfer to a signal, cross gain modulation between signals through pumping light and the like. However, in the amplifying operation by backward pumping, there is a transient response characteristic, which depends on the length of the Raman amplification fiber 1, unlike in the case of forward pumping.
FIGS. 2A and 2B show the output power response waveforms of a Raman optical amplifier. In FIGS. 2A and 2B, the output power of other channels obtained when a prescribed channel of multi-wavelength light in which a plurality of signal channels with a different wavelength each is added/deleted (or stop) being used. It is assumed that a channel that is added/deleted is called “ON/OFF channel”, and the other channels are called “remaining channels”. It is also assumed that pumping power is constant. Furthermore, it is assumed that this Raman optical amplifier is used in a gain-saturated area.
If an ON/OFF channel is deleted when a Raman optical amplifier is used in the gain-saturated area, the saturation level becomes low and the gain becomes high. Therefore, the output power of the remaining channels increases. In FIGS. 2A and 2B, an ON/OFF channel is deleted at time T=50 μs.
In the case of backward pumping, as shown in FIG. 2A, the fluctuations in output power of the remaining channels that are caused due to the addition/deletion (or the stoppage) of use of the ON/OFF channel takes a prescribed response time. This response time depends on the length of the Raman amplification fiber 1 and is approximately twice as long as the fiber propagation time of signal light or pumping light. However, as shown in FIG. 2B, in the case of forward pumping, the output power of the remaining channels fluctuates in a very short time.
FIGS. 3A and 3B show the output power response waveforms of a Raman optical amplifier obtained when inter-signal-light Raman effect occurs. In FIGS. 3A and 3B, the fluctuations in output power of the remaining channels that are caused when an ON/OFF channel is added are shown. Here, it is assumed that pumping power is constant.
In this case, if an ON/OFF channel is added, the output power of the remaining channels varies by two steps of fluctuation speed. Here, the waveform of the output power of the remaining channels varies depending on the wavelength of the ON/OFF channel to be added. For example, if the wavelength of the ON/OFF channel is shorter than that of the remaining channels, as shown in FIG. 3A, the output power of the remaining channels increases rapidly and then varies with a prescribed response time, when that ON/OFF channel is added. However, if the wavelength of the ON/OFF channel is longer than that of the remaining channels, as shown in FIG. 3B, the output power of the remaining channels falls rapidly and then varies with a prescribed response time, when that ON/OFF channel is added. Similarly, the output power of the remaining channels varies by two steps of fluctuation speed, when an ON/OFF channel is deleted.
As described above, in a WDM transmission system assuming that the number of channels is changed during operation, the optical level of the remaining channels fluctuates every time a channel is added/deleted. Furthermore, if the wavelength of a channel to be added/deleted is different, the optical power of the remaining channels varies depending on the wavelength. For this reason, in such a transmission system, it is not easy to manage the transmission characteristic of signal light.
As a method for solving the problem described above, a method of maintaining the gain of a Raman optical amplifier constant by dynamically controlling pumping light is known.
FIG. 4 shows the configuration of a Raman optical amplifier with a function to dynamically control pumping light. In this configuration, the Raman amplifier fiber 1, the pumping light source 2 and the WDM coupler 3 has been already described in FIG. 1.
An optical coupler 4 splits part of the signal light amplified by the Raman amplification fiber 1 and guides it to a photo-receiving device (PD: Photo Diode) 5. The photo-receiving device 5, which can be realized by a photo diode or the like, generates an electrical signal indicating the optical power of the signal light split by the optical coupler 4. A control circuit 6 monitors the optical power of signal light that is amplified by the Raman amplification fiber 1 based on the output of the photo-receiving device 5. Then, the control circuit 6 controls the output power of the pumping light source 2 in such a way that the output power of the signal light may be maintained constant.
As described above, in the Raman optical amplifier shown in FIG. 4 maintains the output power of signal light constant by performing feedback control using information indicating optical output power as a feedback signal. However, there is a limit to the improvement in speed of a feedback system. Therefore, the Raman optical amplifier shown in FIG. 4 has also the following problems.
(1) If an ON/OFF channel is added/deleted in a situation where Raman amplification between signal lights is not negligible, as shown in FIGS. 3A and 3B, the output power of the remaining channels promptly varies. Therefore, control by a feedback system cannot catch up with the variation. Thus, it is practically impossible to suppress the level fluctuations of the remaining channels due to Raman amplification between signal lights by feedback control.
(2) As described in FIGS. 2A and 2B, the output response characteristic of a backward pumping type Raman optical amplifier depends on the length of a Raman amplifier fiber. For this reason, the improvement in speed of the feedback system may disturb the control (there is oscillation, etc.) due to the setting error of a time constant, if flexibility in the design of the feedback system (gain, response time, etc.) is attempted taking a replace of a Raman amplification fiber with another into consideration.
(3) If a plurality of pieces of pumping light with a different wavelength each is supplied, high-speed feedback control must be performed taking into consideration the power balance between the plurality of pieces of pumping light. Therefore, a complex control algorithm is needed.
(4) Since Raman effective length is pretty long, an amplification response time is subject to a fiber propagation time. In particular, in the case of forward pumping, there is a control delay due to the propagation time.
As described above, in the conventional Raman optical amplifier, if the number of the wavelengths of multi-wavelength light changes when the multi-wavelength light is amplified, it is difficult to suppress the fluctuations in optical level of each segment of signal light included in the multi-wavelength light. In particular, when a Raman optical amplifier is used in gain-saturated area or when Raman amplification induced power tilt is not negligible between signal lights, it is very difficult to suppress the fluctuations.